This invention relates to molds for rotational casting of plastic articles, and more particularly to such rotational casting molds having a construction to prevent damage to the molded product from gas trapped between the mold sections when they are joined.
One conventional technique of rotational casting is to provide two mating mold halves having flat faces which can be bolted or otherwise secured together to form a mold cavity. When the two halves are joined, what is known as a "parting line" is formed. This parting line is at the line of intersection between the two mold halves. Normally, immediately adjacent to the parting line are flat faces which abut and sealingly engage each other to maintain essentially a fluid tight interior within the mold cavity. A tongue and groove arrangement is provided in many cases to accurately align the two mold halves.
In the rotational casting process some type of end cover is provided, and the charge is introduced into the mold halves; thereafter the mold is rotated and heated. During this heating process, if there is any air trapped between the two faces, the heating thereof causes expansion of this air; also, expansion of the mold occurs. The expansion tends to drive the air out from between the sealing faces. Unfortunately, in many instances, the air is driven inwardly, rather than outwardly, which often results in a defect of the rotationally cast article formed within the mold along the parting line, which defect takes the form of a raised bubble either partially or entirely distorting the wall of the formed part, making it either of inferior quality or actually unuseable. Even providing very carefully machined and tight fitting sealing faces on the two mold halves does not completely eliminate this problem, and in many instances, it is a very serious problem causing a substantial amount ot scrap.